src: replace unreachable code with static_assert

This function base64-decodes a given JavaScript string to obtain the secret key, whose length must not exceed INT_MAX. However, because JavaScript strings are limited to v8::String::kMaxLength chars and because base64 decoding never yields more bytes than input chars, the size of the decoded key must be strictly less than v8::String::kMaxLength bytes. Therefore, it is sufficient to statically assert that String::kMaxLength <= INT_MAX (which is always true because String::kMaxLength itself is an int).

Aside from being unreachable, Coverity considers the current code "suspicious" because it indicates that buffers larger than INT_MAX might actually be allocated.